This invention relates to a method for reducing the concentration of NH.sub.3 contaminants found in gaseous fuels. More particularly, this invention relates to the pre-combustion injection of NO into coal gas to convert NH.sub.3 to N.sub.2.
Because of the world-wide energy situation, the United States is returning to coal as a significant energy source. The direct utilization of coal poses many problems, most of which can be overcome by gasification. However, a significant problem remains with the production of air pollutants generated during the combustion of coal gas. These pollutants are generated from organically bound coal-sulfur and coal-nitrogen.
The fuel-sulfur is generally converted to H.sub.2 S and the fuel-nitrogen is primarily converted to NH.sub.3 with some HCN in the gasificaation process. Not only are these species relatively toxic and highly corrosive, but they are efficiently oxidized to SO.sub.x and NO.sub.x during the subsequent coal gas combustion process. SO.sub.x and NO.sub.x represent the various oxides of sulfur and nitrogen respectively. These particular combustion effluents have proved to be a major source of air pollution when discharged into the atmosphere. Therefore, a major problem associated with the combustion of coal gas is that federal NO.sub.x and SO.sub.x emission standards will be exceeded.
There have been considerable efforts in the art to find effective ways to remove these particular pollutants from coal gas or from the subsequent effluents of coal gas combustion. The advances have centered around, most particularly, fuel-sulfur derivatives. In particular, fuel-sulfur which is converted to hydrogen sulfide (H.sub.2 S) in coal gas is a chemical species which is much easier to remove than the post-combustion product SO.sub.2. While much of the advances have been made with regard to fuel-sulfur, there remains a significant pollution problem with the formation of NO.sub.x from the gasification and subsequent combustion of coal-nitrogen. Although some of the sulfur related systems also remove some of the ammonia, these systems are not effective in selectively removing ammonia or the subsequent NO.sub.x produced pollutants. It has been calculated that NO.sub.x production from the residual NH.sub.3 will exceed either present (215 g NO.sub.x /10.sup.6 kJ), or future federal standards (39 g NO.sub.x /10.sup.6 kJ) for NO.sub.x emission. It has been reported, for example, that low temperature H.sub.2 S removal systems leave 100-500 ppm of NH.sub.3 in coal gas and that this could lead to the production of about 43-258 g NO.sub.x /10.sup.6 kJ. See Folsom, et al., "Environmental Aspects of Low-Btu Gas-Fired Catalytic Combustion", Proceedings-Third EPA Workshop on Catalytic Combustion, EPA Publication EPA-600/7-79-038, pp. 345-383 (1979). Furthermore, high-temperature H.sub.2 S removal systems do not affect the 400-4000 ppm of NH.sub.3 present in the coal gas, and these levels could produce in the range of 215-2066 g NO.sub.x /10.sup.6 kJ. Furthermore, it should be noted that there are cost and efficiency penalties associated with the low-temperature H.sub.2 S removal systems.
Methods for controlling NO.sub.x emissions for fuel nitrogen have been studied extensively. Most of the combustion modifications which are successful in treating thermal-NO.sub.x (oxides of nitrogen formed from the oxidation of atmospheric N.sub.2) adversely affect fuel-NO.sub.x (oxides of nitrogen formed from the oxidation of NH.sub.3 or other fuel-nitrogen species). Water injection, increased mixing intensity, and inert addition all increase fuel-NO.sub.x. Furthermore, studies related to flue gas recirculation have been inconsistent. For example, Brown, et al., "Low NO.sub.x Combustion: the Effect of External Flue Gas Recirculation on the Emissions of Liquid Fuel Combustion", Combustion Institute European Symposium, Academic Press, New York, pp. 487-492, (1973), have reported an increase in NO.sub.x emission. In a study by Turner, et al., "Influence of Combustion Modification and Fuel Nitrogen Content on Nitrogen Oxides Emissions from Fuel Oil Combustion", AIChE Symposium Series, 68, 126, pp. 55-65, (1972), there was reported no effect on fuel-NO.sub.x subjected to flue gas recirculation. However, in contrast, Martin, et al., "An Investigation of the Conversion of Various Fuel Nitrogen Compounds to NO.sub.x in Oil Combustion", A.I.Ch.E. National Meeting, Atlantic City, (August, 1971), have reported a decrease in fuel-NO.sub.x related to flue gas recirculation. Only staged combustion as reported by Gibbs, et al., "The Influence of Air Staging on the NO Emissions from a Fluidized Bed Coal Combustor,"16th Symposium (International) on Combustion, the Combustion Institute, Pittsburgh, pp. 461-474, (1976) and "Catalytic Combustion" as reported by Folsom, et al., supra, have been shown to decrease both thermal-NO.sub.x and fuel-NO.sub.x.
The inability of most combustion modifications to control both thermal-NO.sub.x and fuel-NO.sub.x is one reason for interest in stack gas treatment techniques. The technique of most relevance to the present invention involves ammonia injection into the flue gas. For example, Lyon, U.S. Pat. No. 3,900,554, Aug. 19, 1975, discloses a post-combustion method wherein NH.sub.3 reacts selectively with NO to form primarily N.sub.2. However, in practice, trace amounts of N.sub.2 O, NH.sub.3, and HCN may be emitted. This concept of NO reduction with NH.sub.3 has been successfully applied to an oil-fired boiler and a gas-fired furnace, yielding up to a 70% decrease in NO.sub.x emission. See, for example, Exxon, "A Way to Lower NO.sub.x in Utility Boilers", Environmental Science and Technology, 11, 3, pp. 226-228 (1977).
The present invention provides a method for converting the NH.sub.3 to N.sub.2 before combustion thereby precluding formation of NO.sub.x in subsequent combustion operations. This process has several advantages over other fuel- NO.sub.x control techniques. Most importantly, N.sub.2 is oxidized at less than 1% efficiency to NO.sub.x during combustion. Therefore, the present inventive process anticipates lowering post-combustive NO.sub.x by up to two orders of magnitude relative to untreated systems.
The present invention applies a NH.sub.3 --NO chemical reaction to the pre-combustion control of fuel-nitrogen in coal gas. This invention contemplates that NO injected into a stream or quantity of coal gas converts the NH.sub.3 contaminants to N.sub.2. This process is beneficial from several perspectives.
First, the end product of the process, N.sub.2, is nontoxic and noncorrosive. This characteristic is especially important if coal gas is to be substituted for natural gas for domestic use, but it is also attractive for a coal gasifier-electric power plant combination.
Second, and most importantly, N.sub.2 is more difficult to oxidize than is NH.sub.3. At combustion temperatures, N.sub.2 is converted to NO.sub.x with an efficiency of conversion of less than 1% compared to NH.sub.3 which can be oxidized to NO.sub.x with an efficiency of up to 100%.
A further advantage of this process is that the conversion of NH.sub.3 to N.sub.2 may take place as the coal gas exits from the gasifier. The temperature of the coal gas as it exits from the coal gasifier is sufficient to drive the conversion reaction without additional heat induction.
A fourth advantage of this process is that it requires no modifications to the combustion system; a situation which often requires high cost equipment fabrication.